Targeted radiopharmaceuticals resolve an image of diagnostic interest or deliver a therapeutic radioisotope to an area of interest by binding or localizing selectively to a site within the body. For various diagnostic and therapeutic applications, chelators that bind a metal radionuclide and are linked to a targeting molecule have been employed with varying degrees of success. Such chelators often have a region incorporating four or more donor atoms that form five- or six-membered rings appropriate for high affinity radionuclide binding. Typical metal radionuclides used for diagnostic imaging agents include 99mTc, 64Cu, 67Cu, 97Ru, 109Pd, 198Au, 199Au, 111In, in their various chlorides, oxides or nitrides. Typical metal radionuclides used for radiotherapeutic applications include 186Re, 188Re, 111In, 166Ho, 105Rh, 149Pm, 153Sm, 177Lu, 90y, 203Pb, 212Pb and 212Bi in their various chlorides, oxides or nitrides. Numerous types of molecules have been employed as targeting molecules, including polyclonal and monoclonal antibodies and fragments thereof, proteins and peptides, especially those capable of binding with specificity to cell surface receptors (generally referred to as “receptor-binding ligands”). When labeling peptide and protein-based targeting agents, the chelator is ideally also peptide-based, so that the chelator-targeting molecule conjugate can be synthesized using peptide synthesis techniques. For example, U.S. Pat. Nos. 5,662,885; 5,780,006; and 5,976,495 (each of which is incorporated herein by reference in its entirety) disclose chelators that bind metal radionuclides, can be coupled to targeting agents capable of localizing at body sites of diagnostic and therapeutic interest, and are peptide analogs designed structurally to present an N3S configuration capable of binding oxo, dioxo and nitrido ions of, for example, 99mTc and 186/188Re. Moreover, peptidic cores that chelate isotopes of Tc and other diagnostic and therapeutic radionuclides are known; most of this work has focused on using peptides derived from natural amino acids of the form NH2CHRCOOH.
Despite the many advances in diagnostic imaging, several obstacles are routinely encountered in this field. One of the key problems is the development of useful formulations for the preparation of the targeted radiopharmaceutical. One problem frequently encountered is that many formulations require a high concentration of the targeted chelating ligand in reactions for the preparation of a targeted radiopharmaceutical to assure high yields of the desired complex. These formulations produce radiopharmaceutical preparations with significant amounts of “free” targeted chelating ligand that has not been chelated to the radiometal. For many applications this “free” ligand is undesirable; thus, it must be separated from the labeled ligand using chromatographic techniques such as High Pressure Liquid Chromatography (HPLC) prior to further use. This is often necessary where the targeting molecule attached to the chelator is, for example, an agonist, and exhibits biological activity when it binds to a target receptor. Such biological activity is undesirable in a diagnostic compound and may also be undesirable in a therapeutic one. In addition, receptor-binding ligands that are not complexed with a radionuclide may compete at the target receptor with those ligands that are complexed with a radionuclide, resulting in poor targeting of the radionuclide complex and poor diagnostic or therapeutic characteristics.
The targeted radiopharmaceutical with the structure shown in FIG. 1 is an example. The uncomplexed receptor binding ligand (or targeted chelating ligand) used to make this complex contains a tripeptide N3S chelator [(N-(Me2)-Gly-Ser-Cys(Acm)] that forms a complex with Tc(V)O, losing the acetamidomethyl (Acm) protecting group in the process. The chelator sequence is linked to the N-terminus of an octapeptide targeting molecule, pGlu-Trp-Ala-Val-Gly-His-Leu-Met-NH2 derived from bombesin, via a linker of Gly-aminovaleric acid. Both this ligand and the Tc complex formed from it are agonists that are known to bind to Gastrin Releasing Peptide (GRP) Receptors with high affinity. See, e.g., U.S. Pat. No. 6,200,546, which is hereby incorporated by reference in its entirety. Clinical studies that were performed with this compound, e.g. those described by Van de Wiele et al. (European Journal of Nuclear Medicine, Vol. 27, No. 11, 2000 p. 1694 (which is hereby incorporated by reference in its entirety) were prepared using the following prototype 4-vial kit. To each of 2 vials, each containing 100 μg of ligand was added 0.1 mL of stannous chloride (2 mM), 0.1 mL of sodium gluconate (60 mM), 1850-2035 MBq (50-55 mCi) of 99mTcO4— in 0.3 mL of 0.9% sodium chloride, and 0.5 mL of sodium chloride. After 35 min. in a boiling water bath, the pooled reaction mixtures were injected on an HPLC system and purified in order to separate labeled from unlabelled peptide, followed by terminal sterilization. The overall yield from this radiosynthesis was ˜30%, with a radiochemical purity after purification of >90%. The purified compound could be stored at 4° C. for only up to 2 hours. This procedure, although useful to provide initial evidence of the potential clinical utility of the compound, is not commercially viable because it uses a 4-vial frozen kit, requires preparative HPLC (i.e., instrumentation that is not typically available in nuclear medicine units) to remove excess ligand prior to injection, and does not provide a sterile product except by terminal sterilization. In addition, it uses >100 mCi of 99mTcO4− to make a patient dose, requires the use of generator eluant that is 2 hours old or less, and the purified product can only be used for 2 hours after isolation.
It would be highly advantageous to have a formulation for the preparation of this and other receptor-binding targeted radiopharmaceuticals that, among other things, can be used clinically (e.g., directly injected) without the need for HPLC purification. The present invention addresses this need and other problems in the art.